A stent device is a tubular vascular implant that has structure able to support a segment of a blood vessel or other anatomical lumen against collapse, while allowing blood or other bodily fluid to flow through the lumen of the stent device. The stent device is collapsed radially for delivery so that the low profile aids access to the target site. The stent device is delivered with a delivery system to the site where a diseased segment of blood vessel is located and deployed there to support the blood vessel against radial collapse. The stent device is advanced to the site in the collapsed configuration and expanded to contact the inner wall of the blood vessel upon deployment. The delivery system generally comprises an inner catheter to which the stent device is mounted and an outer sheath for constraining the stent device in the collapsed configuration.
There are stent devices that require forced expansion such as by inflating a balloon inside the lumen of the stent device and self-expanding stent devices that are so made that they automatically expand to the radially expanded configuration once given the radial freedom to do so; that is once the stent device is unconstrained by the outer sheath of the delivery system. It is with this latter type of stent device that the present disclosure is primarily concerned.
A stent device includes a tubular framework that is resistant to radial compression so that the blood vessel is maintained open. The stent device may include a cover on the inner and/or the outer surface of the framework, in which case the stent device is often termed a stent graft. If the framework is without inner and outer coverings, the stent device may be labelled a bare stent. Primarily, although not exclusively, the present disclosure is to do with bare stents.
One suitable material for making the framework of the stent device is the nickel titanium shape memory alloy known as NITINOL. Such stents may be put into a collapsed configuration at a low temperature and a memory of a radially expanded configuration is maintained. The nickel titanium material is biologically compatible. The NITINOL stent device returns to the expanded configuration between room temperature and body temperature.
A self-expanding stent device is subjected to axial forces during loading of the stent device into an outer sheath of a delivery system and also during deployment of the stent device from the outer sheath of the delivery system to a site of a vascular lumen where it is to be implanted. During these procedures, the stent device is held axially in position by a delivery pusher or a loading mandrel and the outer sheath is moved axially relative to the stent device and the delivery pusher or the loading mandrel. The delivery pusher and the loading mandrel may be the same element and used both for the loading procedure as well as the deployment procedure. It is with the loading procedure that the present disclosure is concerned, and so we will refer to a loading mandrel, although such a device can in some applications also be used as a delivery pusher.
During the loading procedure, the stent device is crimped by a crimping head into a collapsed configuration and moved into the outer sheath of the delivery system. To move the stent device longitudinally from the crimping head to the outer sheath, frictional forces occur between an inner surface of the crimping head and the outer surface of the stent device. Further, as the transfer proceeds, the outer surface of the stent device will frictionally drag against an inner surface of the outer sheath as it is advanced therewithin.
Therefore, as stent device slides through the crimping head and the outer sheath, drag forces on the stent device from the inner surfaces of the crimping head and the outer sheath translate to axial forces on the stent device. These forces can risk axial damage or buckling of the stent device, as is discussed further below.
U.S. Pat. No. 7,316,147 discloses the use of a pushing mandrel to move the stent from the crimping head into the outer sheath of the delivery system once the diameter size of the stent device has been reduced and crimping is thus complete. The pushing mandrel engages against a proximal end of the stent device. For short, axially strong stents, this design is fine. A particular application may require longer stent devices that are desirably flexible so that the tortuous passageways of the vascular system can be traversed. Flexibility and axial strength present a trade-off in properties, where a more flexible stent device is an axially less strong one. In loading device designs, such as the ones disclosed in U.S. Pat. No. 7,316,147, where a mandrel pushes against a proximal end of a stent device, there is a greater risk with more flexible stent devices of deformation in the longitudinal direction as friction from the inner surfaces of the crimping head and the outer sheath on the stent device induces axial forces that are focused at the proximal end of the stent.
International patent publication number WO 2005/070335 recognised that a problem exists whereby buckling of the start device is caused because the longitudinal force exerted on the stent device by the pushing mandrel to expel it from the crimping head is greater than the column strength of the stent device. The document discloses to use fluid as a boundary layer between blades of the crimping head and the stent device as a friction reducing agent, thereby reducing the longitudinal stress placed on the stent device during transfer from a crimping head to an outer sheath of a stent device delivery system.
International patent publication number WO 2004/096091 proposes to reduce the risk of longitudinal buckling of a stent device during loading by distributing longitudinally the engagement between the stent device and the loading mandrel. This is achieved by the provision of a loading mandrel with protrusions and recesses along an outer surface, where the protrusions are embedded within an inner cover layer, made of expanded polytetrafluouroethylene (ePTFE), of the stent device. The embedded protrusions provide a “form fit” between the stent device and the start device pusher, which means that as the crimping head or outer sheath is moved relative to the stent device, resultant axial forces on the stent device are effectively distributed along it. This publication teaches the use of the inner cover of a stent device to distribute axial loading forces. Bare stents, however, also have application in supporting vascular lumens and a method of effectively distributing loading forces for such stent devices is desirable.
As stent devices grow in length, the frictional forces from the crimping head on the stent device also increases, thereby requiring the start device to be strongly secured on the loading mandrel. Furthermore, and particularly with more flexible stent devices, the risk of buckling of the stent device during loading is desirably reduced by uniformly distributing axial forces along the stent device.
It is, therefore, an object of the present invention to provide a method of loading a stent device, particularly a bare stent, into a stent device delivery system, whereby axial forces on the stent device during transference of the stent device from the crimping head to the outer sheath of the delivery system are effectively distributed uniformly along the stent device.